A membrane carburetor is disclosed in U.S. patent application Ser. No. 08/325,939, filed Oct. 19, 1994. Here, the main nozzle path between the control chamber and the venturi section of the intake channel is defined by a first fixed throttle as an uncontrolled path and by a second fixed throttle as a controlled path. The fuel throughput of the controlled path can be changed with the aid of an adjusting screw by the operator. The inflowing fuel quantity is limited by the fixed throttle also for the fully opened controlled path. The total flow of the fuel quantity, which enters into the venturi section via the main nozzle path, is therefore limited so that the maximum fuel-mass flow can be set at the factory for the full-load case so that statutory exhaust-gas values can be maintained.
The known solution has been proven in practice but introduces a considerable complexity when forming the controlled path. This is so because the fuel-mass flow, which flows via the controlled path, is relatively slight compared to the fuel mass flowing through the uncontrolled path. For this reason, a highly precise control valve musk be provided for obtaining a sensitive control. In a control valve of this kind, a mixture adjustment can be made manually which is orientated to the user in order, for example, to obtain high rpm in the full-load case or good acceleration performance.
In practical use, the internal combustion engine is adjusted to the external conditions at the work site and this is time consuming and delays the operational readiness of a work apparatus. If it is further considered that catalytic converters are also utilized in motor-driven chain saws for reducing exhaust-gas emissions, then the air/fuel mixture supplied to the engine must be adapted in order not to deteriorate the operability of the catalytic converter.